Inkjet printing of electrically conductive patterns has been known for some time. The printing process includes the distribution of ink containing conductive particles, polymers, or precursors, across a surface of a substrate. In some cases, a post-treatment such as sintering or plating is necessary to achieve the desired conductivity. The electronics industry uses such inks to form conductive traces on substrates carrying different components. The conductive particles used in ink formulations are of different sizes and depend on the type of printing used. Ink formulations used in inkjet printing usually have properties and particle sizes suitable for ejection from an inkjet head.
Recently, conductive metal micron and sub-micron particles have become of use in the formulation of conductive inkjet inks. Suitable metal particles are silver, gold, copper, palladium, nickel, platinum, and various silver alloys. The size of the particles varies from few microns to around 5 nm. Silver is a precious noble metal with favorable properties such as the highest electrical and thermal conductivity and the lowest contact resistance of all metals, and for this reason it is the most widely used material for conductive inks used in printable electronics. It has a high conductivity, excellent environmental stability, and a lower cost than other noble metals including gold.
Metal particle-loaded inks also find use for decorative printing purposes. Different color and light reflecting or dispersing patterns could be printed using such inks. For example, gold particle-loaded inks faithfully reproduce gold color. Particles in the range of 5-50 nm can produce vivid colors (e.g. red from gold and yellow from silver) on account of their strong surface plasmon absorption band. When the decorative patterns are printed digitally, for example, using inkjet printing methods, the inks should have properties suitable for ejection from an inkjet head.
Cost is a consideration in both conductive and decorative pattern printing, since the production of metal particles cannot be accomplished by typical milling procedures, so more technically sophisticated and expensive methods are required to produce the particles, such as condensation from vapor, precipitation from solution, or etching from an alloy. This results in inks that are more expensive than regular pigment-loaded inks, and are also more expensive than the silver pastes that can be used e.g. for screen-printing and conductive adhesives.